The present invention relates generally to integrated circuits, and more particularly, to antifuse circuits and methods for operating them.
Integrated circuits are interconnected networks of electrical components fabricated on a common foundation called a substrate. The electrical components are typically fabricated on a wafer of semiconductor material that serves as a substrate. Various fabrication techniques, such as layering, doping, masking, and etching, are used to build millions of resistors, transistors, and other electrical components on the wafer. The components are then wired together, or interconnected, to define a specific electrical circuit, such as a processor or a memory device.
Fusible elements are employed in integrated circuits to permit changes in the configuration of the integrated circuits after fabrication. For example, fusible elements may be used to replace defective circuits with redundant circuits. Memory devices are typically fabricated with redundant memory cells. The redundant memory cells may be enabled with fusible elements after fabrication to replace defective memory cells found during a test of fabricated memory devices. Fusible elements are also used to customize the configuration of a generic integrated circuit after it is fabricated, or to identify an integrated circuit.
One type of fusible element is a polysilicon fuse. The polysilicon fuse comprises a polysilicon conductor fabricated to conduct electrical current on an integrated circuit. A portion of the polysilicon fuse may be evaporated or opened by a laser beam to create an open circuit between terminals of the polysilicon fuse. The laser beam may be used to open selected polysilicon fuses in an integrated circuit to change its configuration. The use of polysilicon fuses is attended by several disadvantages. Polysilicon fuses must be spaced apart from each other in an integrated circuit such that when one of them is being opened by a laser beam the other polysilicon fuses are not damaged. A bank of polysilicon fuses therefore occupies a substantial area of an integrated circuit. In addition, polysilicon fuses cannot be opened once an integrated circuit is placed in an integrated circuit package, or is encapsulated in any manner.
Another type of fusible element is an antifuse. An antifuse comprises two conductive terminals separated by an insulator or a dielectric, and is fabricated as an open circuit. The antifuse is programmed by applying a high voltage across its terminals to rupture the insulator and form an electrical path between the terminals.
Antifuses have several advantages that are not available with fuses. A bank of antifuses takes up much less area of an integrated circuit because they are programmed by a voltage difference that can be supplied on wires connected to the terminals of each of the antifuses. The antifuses may be placed close together in the bank, and adjacent antifuses are not at risk when one is being programmed. Antifuses may also be programmed after an integrated circuit is placed in an integrated circuit package, or encapsulated, by applying appropriate signals to pins of the package. This is a significant advantage for several reasons. First, an integrated circuit may be tested after it is in a package, and may then be repaired by replacing defective circuits with redundant circuits by programming selected antifuses. A generic integrated circuit may be tested and placed in a package before it is configured to meet the specifications of a customer. This reduces the delay between a customer order and shipment. The use of antifuses to customize generic integrated circuits also improves the production yield for integrated circuits because the same generic integrated circuit may be produced to meet the needs of a wide variety of customers.
Despite their advantages, the use of antifuses in integrated circuits is limited by a lack of adequate circuitry to support the programming and reading of the antifuses. There exists a need for improved circuits and methods for programming and reading antifuses in integrated circuits.
The above mentioned and other deficiencies are addressed in the following detailed description. According to embodiments of the present invention several support circuits have elements to program and read antifuses. An external pin is coupled to a common bus line in an integrated circuit to deliver an elevated voltage to program antifuses in a programming mode. An antifuse having a first terminal coupled to the common bus line is selected to be programmed by a control transistor in a program driver circuit coupled to a second terminal of the antifuse. The program driver circuit has a high-voltage transistor with a diode coupled to its gate to bear a portion of the elevated voltage after the antifuse has been programmed. The program driver circuit also has an impedance transistor between the high-voltage transistor and the control transistor to reduce leakage current and the possibility of a snap-back condition in the control transistor. A read circuit includes a transistor coupled between a read voltage source and the second terminal to read the antifuse in an active mode. The common bus line may be coupled to a reference voltage through a common bus line driver circuit in the active mode to pass current to or from the read circuit. The common bus line driver circuit has a control transistor and a high-voltage transistor with a diode coupled to its gate to bear the elevated voltage on the common bus line during the programming mode. The read circuit may have a latch circuit to latch a state of the antifuse in a sleep mode. A floating well driver logic circuit raises the voltage potential of wells and gate terminals of p-channel transistors in the read circuit during the programming mode to reduce current flow from the common bus line.
The embodiments of the present invention support the programming and reading of antifuses in an integrated circuit, and facilitate all the advantages associated with the use of antifuses in integrated circuits. Other advantages of the present invention will be apparent to one skilled in the art upon an examination of the detailed description.